Replacement oil boiler with propane condensing boiler vs conventional gas boiler

[Jackie123]

Hi,

I think I have to replace my oil boiler which is about 60( I think it is the original boiler) years old as it does no longer meet codes, I cannot get any more oil delivered unless I can fix the problem. Since my water heat is oil and 16 years old it will have to also be replaced. I realize my boiler will have to be replaced someday but this is not the best time to replace it. The boiler is a Hoffman net btu 101250. I have had the boiler cleaned and maintained yearly. It is not the boiler but the exhaust pipe that is too close to the outside brick wall and inside dividing wall which is load bearing. If I removed the studs on the outside wall it would be about 5 inch from brick wall which would be alright but the interior load bearing wall is a problem. I need to be 10 inches from this wall. I can replace the studs with metal supports but I still will be too close to the wall. The furnace cannot be moved as it was built in place. The oil company just said I need a new furnace and did not make any other suggestions. I have been told the pipe has to be 10 inches from any combustible material. I spend about $3000.00 on oil last year. I am told that I will save about $1000.00 a year and it is a long payback period, even if I do save that much a year.
I am looking at changing to propane(no natural gas) from oil. I have been given 2 options and have gotten 3 quotes, all somewhat different.

- IBC 115 condensing boiler and Eco-tank indirect wh-most expensive about $10,000.
-normal boiler(LarrsJVS boiler. Eco WH) with indirect hot water heater- about $9000.00

This is a major investment and I want to get it right. I have a brick house about 60 years old, about 1000 sq. feet., with OK windows and good insulation in attic. I have baseboard fin rads. I have read that you do not get the full efficiency from a condensing boiler with this type of baseboard heater as it requires higher temperatures than 137 where condensing starts. I have also ready that installation really matters with the condensing boiler and they can break more. Also is it correct that you do not need a thermostat with a condensing boiler as it has been included in some quotes but not in others. I have also been told by one contractor that condensing boilers do not need yearly maintenance which seems odd or thermostat. I have attached a picture of the boiler and pipe.

PS. I have been reading threads on the site and Dana suggested in one case that "a more cost effective solution is to keep the boiler (but adding an intellicon) but to add an R410A refrigerant mini-split heat pump with at least 2-ton (24,000BTU/hr) nominal heating capacity which would cut down your heating cost substantially if used as the primary heat source, using the boiler only as the backup during the bitterest weather.". If I could somehow fix the problem with the vent pipe would this be a possibility with a 60+ year old boiler. No I have spoke to suggested this when I asked about a heat pump.

Any suggestions would greatly appreciated(I am a single woman 60+ and not to handy).
Thanks, Jackie

 

[Dana]

The fin-tube baseboard only needs the higher temp water during the absolute coldest weather. Most modulating condensing boilers have the option of adjusting it's output temperature in response to the temperature outside to take better advantage of condensing efficiency when the conditions aren't as severe. This is referred to as "outdoor reset", and it needs to be programmed/adjusted to fit the particulars of the climate, heat load, and amount/type of radiation.

At temperatures below ~120F the output of fin-tube baseboard is less predictable, so adjusting the outdoor reset curve gets complicated if you try to run it below 120F, but that is a temperature at which the condensing efficiency of propane is QUITE GOOD!

The other issue with fin-tube is that it has very little water volume, and thus very low thermal mass, so it's output at lower water temps needs to be somewhat reasonably matched to the minimum-fire output of the boiler or the boiler will fire in short efficiency robbing bursts that put a lot of wear & tear on the boiler. The minimum fire output of the IBC SL 20-115 is about 18,000-19,000BTU/hr. The output of fin-tube baseboard at 120F is about 200BTU per foot, so to be perfectly balanced you would need about 95' of baseboard, and running it as a single zone, but even 80' would still be OK.

The old boiler has an output of 160,000 BTU/hr and was probably designed for 180F average water temps, where the fin tube emits about 600 BTU/foot, so you probably have at least a couple hundred feet of fin tube, but measure it all up just to be sure. Anything over 80' would be just fine for that boiler, but there are smaller boilers out there with even lower minimum-firing ranges that would be suitable too. Your oil use isn't outrageous, and the oversized boiler was probably running at well under it's nameplate efficiency. Your actual heat load at Hamilton's +1F/-17C 99% outside design temp is probably under 50,000 BTU/hr, and certainly under 100,000BTU/hr, even if the walls are uninsulated double-wythe-no-cavity brick. If you can tell me what that $3000 oil cost was in liters or other volume measurement it's pretty straightforward to place an upper bound on your heat load, and a BTU/foot max on the baseboard (if you tell us how many feet).

If your home has a relatively open floor plan and the heat load estimates comes in at or under 30,000BTU/hr (it might), it may be cheaper and better to mothball the baseboard and heat with mini-split air source heat pumps, which would cost less than half as much to heat with than condensing propane, and would be cheaper to install too. There are a few models that are designed to heat at outdoor temps of -25C or lower (-25C is the all time record low temp recorded for Hamilton, according to Weatherspark.com datasets.) They are very popular as the solution for people in super-insulated houses, but it's a solution that also works well in smaller houses off the gas-grid as long as the house is not all chopped up into tiny doored-off rooms.

A pair of the 1-ton Mitsubishi MUZ/MSZ FE12NA would run about $7KUSD (installed) in my neighborhood, with a combined output at -17C of about 30,000BTU/hr. The slightly bigger -FE18NA puts out about 20,000BTU/hr @ -17C, at an installed cost of about $4-4.5KUSD per. The alternative would be the Fujitsu AOU-xxRLS2-H series. (I don't have installed pricing info on those yet, but it won't be dramatically more than the competition.) But since they are point-source heat, doored off rooms won't stay up to temp with the doors closed, and some supplemental resistance heating for those rooms may be called for during the coldest weather.

With a ductless heat pump solution you'd still have to do something else for hot water, but electric hot water heaters don't cost much to install, and your power utility rates aren't outrageously high.

 

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[Jackie123]

Thank you for all your information. I have raised bungalow, 2 bedroom upstairs, bathroom, open concept living room, dinning, kitchen, 65 ft. of baseboard. Down stairs, 2 bedroom, bathroom, family room, furnace/laundry room, only 32 feet of baseboard for a total of 97. The down stairs was finished at a later date than upstairs. The furnace creates a lot of heat that helps keep the downstairs warm. If you close the door down stair I use an small electric heater in the room to keep it comfortable or it gets to cold. Most of the time the doors are left open. I estimate I used about 2890 liters of oil but that includes oil for the hot water. I live on my own and do not use a lot of hot water. Would it be possible to use heat pump. I checked with one of the contractors who installs heat pump and he said I would still require addition heat back heat.

I have been checking into heat shields to put against the wall to make vent meat codes and keep the boiler I have or at least allow more research time. If I could fix the wall should I consider a new boiler anyway as my is so old but seems to work well.

 

[Dana]

With 97' of baseboard at a 120F average water temp (AWT) the fin-tube can emit the full min-modulated boiler ~19,000BTU/hr output of the IBC 20-115, which is GOOD.

If it's controlled as two separate zones you'll still be OK with the zone that's 65', but there may be cycling at low temp on the smaller zone, but there are ways to deal with that if the burns are always TOO short.

If you went with a conventional mid-efficiency gas boiler it's D.O.E. output should be no more than what the fin-tube can deliver with 180F AWT, which is about 60,000 BTU/hr, which is about the smallest of the line for many manufacturers, but if there is one with 50,000BTU/hr-output or even 40,000BTU/hr out it would be better, especially if it's controlled as separate zones.

2890 liters of oil in an 85% efficiency boiler delivers about 88 million BTU into the heating system. It looks like in a typical year Hamilton gets about 6500F heating degree days, so that works out to ( 88,000,000/6500=) 13,538 BTU/F-degree-day, or (13,538/24 hours per day =) 564 BTU per degree-hour. Assuming a cooling/heating balance temp of ~65F/18C (your electricity plug loads and body heat bring it up to 20C when it's 18C outside) and a 1F outside design temp that's (65F-1F=) 64F heating degrees.

So at -17C/+1F it takes (564 x 64F=) 36,100 BTU/hour to keep the place at 20C indoors, or (36,100/97'= ) 372 BTU per hour per foot of baseboard. That means at the outside design temp you would see average water temps of about 150F and the condensing boiler would be running at about 87% efficiency, but at your average mid-winter temp of 25F/-4C you would have only (65F-25F=) 40F heating degrees, for a load of 40F x 564= 22,560 BTU/hr the boiler WOULD be modulating, MOST of the time (yay!). The BTU/foot then works out to 22,560/97'= 232 BTU/hr per foot, which can be delivered with an AWT of about 125F, which means you would also be condensing MOST of the time! This means the condensing boiler WOULD give you the full benefit, since at warmer temps it could be set up with a minimum output temp of 120F or 125F to limit short-cycling and produce predictable highly efficient output.

But with a design heat load of only ~36K, you can probably heat the whole place comfortably most of the time with with a 1.5 ton or 2-ton mini-split in the open floor plan zone. At my local propane and electricity costs heating with the mini-split would cost well under half what it costs to heat with condensing propane. eg: The 1.5 ton Mitsubishi MUZ/MSZ FE18NA delivers about 22,500 BTU even at -15C/+5F, and more at your average winter temp, so even at your -17C design temp it could be providing nearly all of the heat, at half the cost of heating with propane or oil. Depending on the layout and room by room loads you may be able to heat the upstairs with ductless too, but even with judicioius use of electric cove heaters/panel radiators etc for comfort in those rooms your cost would likely be comparatively quite low. To do a full analysis would require a more careful heat load calculation, and your actual local delivered per-kwh or per-liter costs for fuels.